(A) Field of the Invention
Thermoplastic multifilament feed yarns may be processed into torque yarns and torque yarns may be processed into set yarns. The processing of feed yarns into torque yarns involves the heatsetting of the yarns in a twisted state and under a temperature and tension so as to substantially preclude ductility in the cooled yarn. The reduction of the torque usually follows in either a continuous or discontinuous process, whereby, after the yarn is permitted to cool, the twist is removed from the yarn and the torque is reduced by a second heat treatment. Both heat treatments require carefully controlled tension, and the tension normally employed in the first heat treatment is relatively high, and the tension employed in the second heat treatment is relatively low. In a continuous process, separate tension controls contribute to the expense and complexity of the process; moreover, and this is particularly applicable where the first heat treatment is combined with an orientation drawing step, the speed of the yarn as it passes over the second heater at a relatively low tension requires a relatively long heater plate to effectively reduce the torque. If the tension of the yarn over the second heater were increased to that of the tension over the primary heater, the length of the second heater could be substantially reduced, effecting yet another process conservation measure.
(B) Description of the Prior Art
The production of thermoplastic false-twist crimped torque yarn is well known in the art, as set forth in such teachings as U.S. Pat. No. 2,803,109, one of the so-called "single-heater patents" wherein crimped, wavy or fluffed yarns, highly twisted straight compact yarns or crepe yarns are produced from drawn or undrawn thermoplastic feed yarns by setting a yarn in a twisted state under sufficiently high tension during the actual heating as to preclude substantially any ductility in the finished yarn when cooled.
It is also well known, as taught in U.S. Pat. No. 3,091,912, that such torque yarns can be processed or reprocessed continuously or discontinuously by subjecting the yarn, after cooling and removal of the twist to a controlled degree of tensile stress with or without correlated heat or both and with or without additional twisting, untwisting or false-twisting, thereby to reduce or substantially eliminate the torque.
One of the reasons for the success of partially oriented polyester for draw-texturing was that the high speed spinning in lieu of full or partial orientation drawing on the conventional spin-draw machines permitted increased production by the fiber producer on the speed limiting winder in terms of total denier wound. The next phase of the yarn processing, as was known, could readily absorb the increased speed required to orientation draw the yarn as well as texture it, and while this separation of the orientation drawing from the spinning phase has increased production overall, it has led to considerably faster operation on the texturing machines. Since the orientation drawing takes place during the heatsetting of the twisted yarn, it is the second phase or second heat treatment which must absorb the increased speed created by the drawing of the yarn. This means that when heatsetting torque yarn in a high speed continuous process, the draw rolls normally between the false-twist device and the second heater are operating at a much higher speed than the feed rolls which are upstream of the primary heater. The increased speed with which the yarn passes the second heater requires a longer surface exposure on the second heater to properly set the yarn, and practical limitations upon the length of the second heater are tantamount to production restrictions. Moreover, it is always desirable as a conservation measure to eliminate any of this expensive machinery which can be dispensed with. Capital expenditure for such machinery is extremely large and frequently cannot be justified without a showing of potential for higher production; and any cost reduction is a practical advance in the art.